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Astronomers studying the Coma Cluster of galaxies have discovered that galactic collisions are powerful enough to strip a galaxy of its gas and fling star-forming fireballs out into space.

Galaxy RB199 is the large galaxy on the right, with filaments and fireballs extending out towards the left to a distance of 260,000 light years. Several filaments with blue colour extend linearly from RB199, while red coloured filaments of ionized gas lie further away from the galaxy. Image: Yoshida et al/Subaru Telescope.

It is well known that plants and animals evolve over time to adapt to a changing environment, but now it seems that galaxies evolve too, by changing their shape, size and colour. An ideal laboratory for studying the effects of the environment on galactic evolution are galaxy clusters: dense, rich populations of thousands of galaxies that are bound by strong gravitational forces and hot intergalactic gas. One such galaxy cluster is the Coma Cluster, which contains around 1,000 galaxies and is located 300 million light years away. This provided the focus for a team of researchers from the National Astronomical Observatory of Japan and the University of Tokyo, who used the Suprime-cam on the Subaru Telescope to observe the cluster.

While studying one particular galaxy, Galaxy RB199, the astronomers noticed a thread-like structure stretching out from the galaxy for about 260,000 light-years. The filament contained many young stars surrounded by ionized gas, creating the appearance of fireballs shooting out into space.

Detailed study identified several bright knots connected by blue filamentary structures, which were found to be clusters of young stars weighing the equivalent of about 10 million Suns contained within an area about 3,000 to 6,000 light-years across. The presence of ionized gas implies that active star formation is occurring inside the knots, much more than would normally be expected in this region. The team also noted that the size and the mass of the fireballs indicate they could develop into dwarf galaxies.

Close up of the fireball galaxies of RB199, which are accompanied by ionized red gas. Image: Yoshida et al/Subaru Telescope.

But what was the mechanism that stripped gas from Galaxy RB199 and created the fireballs in the first place? The astronomers came up with a couple of ideas. First, and because the inside of the cluster is so crowded with galaxies they pass by each other and crash into each other, it seemed logical that tidal forces during such encounters could strip gas or stars from the galaxies. But they also postulated that as a galaxy falls into the centre of the cluster, the gravitational forces of the cluster could remove the gas and stars from that galaxy. Both scenarios explain how the gas can be removed from the galaxy, but not the formation of the fireballs.

The team then realised that ram pressure stripping occurs when superheated gas (several tens of million Kelvin) in the cluster and the galaxies collide at high speeds. Previous X-ray observation shows the presence of large amounts of hot ionized gas in the middle of the Coma Cluster, and as RB199 crashes into the centre at a speed of nearly 2000 kilometres per second it causes strong friction with this hot gas. As such, the team concluded that the ram pressure has enough power to strip the gas from the galaxy and create the fireballs, and that this stripped gas then goes on to form stars as it powers through remote space.

"The team is confident that our study of these fireball phenomena leads to a better understanding of the gas stripping processes in galaxy clusters, and the effect of clusters on the evolution of individual galaxies," says Principal Investigator for the project, Dr Michitoshi Yoshida.

While similar phenomena have been observed in galaxy clusters much further away at several billions of light years, those distant cases were interpreted through witnessing the transitional phase of galaxies changing their morphology or colours as they fell into a cluster. The fireballs discovered in this latest observation campaign provide the first sample of such structures in a nearby cluster.

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